Directionally oriented products of reconstituted lignocellulose materials, such as strands, splinters, flakes, particles, fibers, etc., are desirable for structural purposes. The production of directionally oriented products from lignocellulose materials by electrostatic orientation of discrete pieces of the lignocellulosic material are described in U.S. Pat. Nos. 4,287,140 and 4,323,338. In a total system, the lignocellulosic material is dried to a moisture content of between 2-8% on a dry weight basis, blended with a resin such as a urea formaldehyde, or phenolformaldehyde or isocyanate resin, passed into a former which meters approximate amounts of the lignocellulosic material between spaced electrically charged plates for alignment of the particles in the direction of the electric field as they descend by gravity between the spaced plates and are deposited in aligned condition on a mat-receiving surface for later consolidation under heat and pressure in a press.
Orientation of lignocellulosic materials in an electric field is more effective where the lignocellulosic materials have a conductance of at least about 500 femto-Siemens (fS) or 500.times.10.sup.-15 Siemens. As used herein, conductance is defined as the reciprocal of electrical resistance of the particle. It is recognized that conductivity, rather than conductance, may be a more proper measurement; however, conductance was used. Where the term "conductance is used herein, the term refers to measurements which were taken by means of a pair of biased metallic electrical contacts placed at a distance of about 10 mm along the major axis of the elongated particle material pieces. An electric potential, V, was placed between the contacts, and the electric current, I, flowing from one contact through the particle to the other contact was measured.
Conductance G was calculated from the formula: EQU G.times.I/V
Conductivity could be calculated from: EQU .sigma.=GL/(wh),
where L is the distance between the contacts, and w and h are the width and thickness, respectively, of the particle.
Since for particles considered here, the term (L/wh) varied over a range of about 10:1, and the conductance G varied over a much wider range (often in excess of 10,000:1), it was convenient to ignore the effect of particle geometry (L/wh), and report only the conductance. Particle geometries were within a typical range for all particle pieces considered.
Lignocellulosic furnishes from commercial board plants generally include discrete particles with a wide range of conductance, much of which are substantially below the 500 fS limit for adequate orientation. This results in reduced effectiveness of orientation and problems of particles clinging to charged surfaces in the equipment. While conductance can be improved by increasing the moisture content of the lignocellulosic furnish, too high a moisture content in the furnish cannot be tolerated in the pressing operation.
In a commercial board plant where the time lapse between the drying, blending and forming operations is generally only a few minutes, the conductance of the lignocellulosic particles making up the furnish must be brought within the appropriate range for electrostatic orientation very quickly after the drying operation. During a typical drying operation, a portion of the furnish becomes overheated and overdried and rapidly loses conductance as its temperature is reduced after leaving the dryer. A gradual increase in conductance occurs as the furnish equilibrates with the moisture in the environment; however, the time lag to do so is far too long for consideration in a commercial board operation. Application of resin during the blending operation also enhances the conductance of the furnish by increasing the moisture content and by the presence of possible ionic materials in the resin; however, the conductance is generally only improved marginally. Since resin and moisture are not generally well distributed in blending, the conductance of the particles varies over an extremely wide range.
A wood flake having a thickness, for example, of about 0.015 inch, will equilibrate in temperature in about one minute, its conductance increasing with temperature. The moisture content will equilibrate in about one hour with the conductance increasing steeply as the moisture content increases. Heating of wood flakes, for example, to 400.degree. F. decreases the actual hygroscopicity of the wood so that when the flake is returned to a moist atmosphere at room temperature the conductance will increase from below 1 fS to a new equilibrium value in about an hour; however the equilibrium value will be one to two orders of magnitude lower than the measured conductance of the flake before heating. The flake will not recover its former degree of hygroscopicity. Drying wood flakes to a very low moisture content even at room temperature reduces the conductance of those flakes to less than 1 fS with very slow recovery. Flakes which are dried to such a low moisture content and then equilibrated at 38% relative humidity do not reach a suitable range of conductance for adequate electrostatic orientation for several days.
What is needed is a means of ensuring an adequate conductance of fibrous particles for making up consolidated articles, including overdried particles, at the time of forming or before, without increasing the moisture content of the furnish.